1. Field of the Invention
This invention relates to polishing systems and particularly to chemical mechanical polishing systems and methods using fluids to support a polishing pad.
2. Description of Related Art
Chemical mechanical polishing (CMP) in semiconductor processing removes the highest points from the surface of a wafer to polish the surface. CMP operations are performed on unprocessed and partially processed wafers. A typical unprocessed wafer is crystalline silicon or another semiconductor material that is formed into a nearly circular wafer. A typical processed or partially processed wafer when ready for polishing has a top layer of a dielectric material such as glass, silicon dioxide, or silicon nitride over one or more patterned layers that create local topological features on the order of about 1 .mu.m in height on the wafer's surface. Polishing smoothes the local features so that ideally the surface of the wafer is flat or planarized over an area the size of a die formed on the wafer. Currently, polishing is sought that locally planarizes the wafer to a tolerance of about 0.3 .mu.m over the area of a die about 10 mm by 10 mm in size.
A conventional belt polisher includes a belt carrying polishing pads, a wafer carrier head which holds a wafer, and a support assembly that supports the portion of the belt under the wafer. For CMP, the polishing pads are sprayed with a slurry, and pulleys drive the belt. The carrier head brings the wafer into contact with the polishing pads so that the polishing pads slide against the surface of the wafer. Chemical action of the slurry and the mechanical action of the polishing pads and particles in the slurry against the surface of the wafer remove material from the wafer's surface. U.S. Pat. Nos. 5,593,344 and 5,558,568 describe CMP systems using hydrostatic fluid bearings to support a belt. Such hydrostatic fluid bearings have fluid inlets and outlets for fluid flows forming films that support the belt and polishing pads.
To polish a surface to the tolerance required in semiconductor processing, CMP systems generally attempt to apply a polishing pad to a wafer with a pressure that is uniform across the wafer. A difficulty can arise with hydrostatic fluid bearings because the supporting pressure of the fluid in such bearings tends to be higher near the inlets and lower near the outlets. Accordingly, such fluid bearings often apply a non-uniform pressure when supporting a belt and polishing pads, and the non-uniform pressure may introduce uneven removal of material during polishing. Methods and structures that provide uniform polishing are sought.